The US claims to have intercepted Iranian drones before launching missile strikes on what it described as "radar sites" in the south of Iran.
New military exchanges between the United States and Iran have tested the fragile ceasefire, threatening efforts to stabilise the Middle East and restore shipping through the strategically important Strait of Hormuz.
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The latest escalation began on Friday when the US military said it intercepted four Iranian drones heading towards the Strait of Hormuz before carrying out strikes against Iranian coastal radar installations in Goruk and on Qeshm Island.
According to US Central Command (CENTCOM), the drones posed an immediate threat to maritime traffic in the area and it "subsequently struck Iranian coastal surveillance radar sites... to defend against further attacks".
The developments mark a serious challenge to a ceasefire that has largely held since April 8, following a conflict triggered by US and Israeli strikes that killed much of Iran's senior leadership.
Tensions intensified further after air raid sirens sounded in Kuwait and Bahrain, both close US allies in the Gulf. AFP correspondents reported hearing explosions in both countries.
Kuwait's military said its air defence systems were responding to "hostile" missile and drone attacks but did not specify who launched them.
Iran's Revolutionary Guards later said they had launched missiles at what they described as "enemy bases in the area" in response to what they called a US "invasion" of the Iranian islands of Sirik and Qeshm.
CENTCOM said Iran fired seven ballistic missiles towards Kuwait and Bahrain. Six were intercepted, while a seventh failed to reach its target.
"There are currently no reports of harm to US personnel, and Iranian claims of damaging US 5th Fleet headquarters in Bahrain are false," CENTCOM said in a statement.
The renewed violence comes after weeks of difficult diplomatic efforts aimed at ending the conflict and reopening the Strait of Hormuz, one of the world's most important energy shipping routes.
The waterway handles a significant share of global oil and liquefied natural gas exports, making any disruption a concern for international energy markets.